The present invention relates to a process for producing silicone resins and more specifically the present invention relates to a process for producing silicone resins with a viscosity that does not exceed 100 centipoise at 140.degree. C.
Silicone resins produced with the use of a hydrolysis medium of acetone, a water-immiscible organic solvent and water are the basis of the invention disclosed in Merrill Ser. No. 628,001, filed Nov. 3, 1975, which silicone resins produced by such a process are disclosed as useful for molding applications. Such silicone resins produced in accordance with the above disclosure of the above patent application are useful for the production of most molded parts and molding applications as well as other silicone resin applications. However, it was found that when such silicone resins were used to produce molded parts for encapsulating intricate electrical components that at times the molded silicone resin covering would craze or crack during the fabrication and specifically during the final curing of the encapsulated part.
Accordingly, it was found that such silicone resin molding composition did not have as low a coefficient of thermal expansion as would be desired. It can be appreciated that the lower the coefficient of thermal expansion is of the silicone resin molding composition the less it will expand or change shape during the curing steps and accordingly the less the likeliness that it will craze or crack.
As background, it should be appreciated that silicone resin molding compositions usually comprise a silicone resin, a filler and a curing catalyst system, the curing catalyst system usually being some form of lead compounds. However, the portion of the silicone resin molding composition that is most affected by temperature changes so as to increase or decrease in size with a change of temperature is the silicone resin. In prior art compositions, it was usually typical to include in quantity sufficient silicone resin such that there was present 25 to 30% by weight of the composition of silicone resin in order to obtain the proper strength in the encapsulating molding composition.
Accordingly, it was postulated with the above discussion that by decreasing the amount of silicone resin in the molding composition to 15 to 20% by weight of the total composition that the desired low coefficient of thermal expansion could be obtained.
However, the resin produced by the foregoing Merrill process as disclosed in that particular case could not be utilized at such low concentrations since it does not result in a final molded part of optimum tensile strength.
Another difficulty with the silicone resin as disclosed generally in the foregoing Merrill application is that after the filler and catalyst have been mixed into it and prior to being injected into the mold to encapsulate the electrical component, it was found to have too high a viscosity, such viscosity in many cases being in the range of about 600 centipoise or above at 140.degree. C. Such high viscosity resins and molding compositions made it very difficult in the time limit of the automated operation to completely fill the mold with the silicone resin molding composition, and accordingly a sizable number of faulty parts can be produced.
Accordingly, it was also highly desirable to produce a silicone resin of a low viscosity such as a viscosity of 100 centipoise or less at 140.degree. C., such that the silicone resin molding composition could be easily inserted into a mold to completely fill the mold in an automated process.
Accordingly, it is one object of the present invention to provide for a process that produces a silicone resin of low viscosity, that is, a viscosity that does not exceed 100 centipoise at 140.degree. C.
It is another object of the present invention to provide for a process for producing a silicone resin that has good physical properties.
It is an additional object of the present invention to provide for a molding composition that has an exceptionally low viscosity.
It is yet an additional object of the present invention to provide for a molding composition with low coefficient of thermal expansion.
These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.